If converting to 5.0 carbed, it's easy compared to FI. You said you were already swapping K member, so that's taken care of. Other than that, you'll need:
Radiator, (fan stays same, and you can run it switched style, contacts are labeled)
ACC Brackets from 94/95 5.0 (don't use the 93 and down brackets, the fitiment is too tight for those)
EDIT:
Timing Cover/ Water Pump from 94/95: They're different between the years, so you'll need this particular setup in order to run the 94/95 ACC brackets. This timing cover also does not allow you to mount up a mechanical fuel pump, forcing you to go with an electrical pump.
EDIT #2:
Accessories You'll need all the Accessories from a 5.0 car as well. I'm unsure of the differences between the Alternator, so atleast put it in possible buys. You will need power steering pump from a 87-95 car, and it's up to you whether or not you want A/C and the Smog pump. If so, you'll need those from a 94/95 car.
Fuel tank from a carbureted Mustang(83-85 should work, unsure of earlier year designs) (OR you can sump your current 98 tank.. This would be my choice.)
Fuel pump (I would highly recommend going with an electrical pump, especially since it looks like you're putting in a 347. I can give recommendations if you'd like. Give me a price range though)
Fuel Regulator (if going with an electric pump)
Gauges: Either get used to not having a working cluster, and using Autometers around the cabin, or check out the Florida 5.0 clusters.
http://www.florida50.com
Transmission: (I know you mentioned this already)
EDIT #3
Starter: You MAY need a 94/95 starter. The wiring should be the same between your 98 and the 94/95, but I there's a good possibility the starter itself is different. I'd check up on this.
Driveshaft: (for a T-5 transmission)
Exhaust: You'll need headers, like you mentioned, as well as an H or X pipe. You cannot use the 4.6 H/X pipe. Cat-back remains the same.
That's all I can think of off the top of my head... Good luck!
